The present invention relates to fluid flow fields for fuel cells. More particularly, it relates to flow field designs for improving water management and reactant distribution in solid polymer electrolyte fuel cells.
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits.
Fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
A broad range of fluid reactants can be used in solid polymer electrolyte fuel cells and may be supplied in either gaseous or liquid form. For example, the oxidant stream may be substantially pure oxygen gas or a dilute oxygen stream such as air. The fuel may be, for example, substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or an aqueous liquid methanol mixture in a direct methanol fuel cell. Reactants are directed to the fuel cell electrodes and are distributed to catalyst therein by means of fluid diffusion layers. In the case of gaseous reactants, these layers are referred to as gas diffusion layers.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The catalyst layer typically contains ionomer, which may be similar to that used for the solid polymer electrolyte (for example, Nafion(copyright)). The catalyst layer may also contain a binder, such as polytetrafluoroethylene. The electrodes may also contain a substrate (typically a porous electrically conductive sheet material) that may be employed for purposes of mechanical support and/or reactant distribution, thus serving as a fluid diffusion layer.
The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, in addition to good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
In a fuel cell, flow fields are employed for purposes of directing reactants across the surfaces of the fluid diffusion electrodes or electrode substrates. Flow fields are disposed on each side of the MEA and comprise fluid distribution channels. The channels provide passages for the distribution of reactant to the electrode surfaces and also for the removal of reaction products and depleted reactant streams. The flow fields may be incorporated in the current collector/support plates on either side of the MEA (in which case the plates are known as flow field plates) or, alternatively, may be integrated in fluid distribution layers of the electrodes (in which case, the flow field is also the fluid diffusion layer and distributes reactant to the catalyst layer).
In solid polymer electrolyte fuel cells, proper flow field design is important not only for appropriate reactant distribution but also for management of the water produced from the electrochemical reactions of the fuel cell. If reaction product water accumulates within either the reactant flow field channels or the reactant distribution layers in the electrodes, the water may prevent the reactants from accessing the catalyst at the membrane-electrode interface, causing a decrease in fuel cell performance. (The voltage of a fuel cell at a particular current density is a measure of performance. At a given current density, higher cell voltage signifies higher performance.)
For purposes of water management, a higher pressure differential (that is, pressure drop) between a reactant flow field channel inlet and outlet may be used to reduce or eliminate the accumulation of product water; see, for example, U.S. Pat. Nos. 5,260,143, 5,366,818, and 5,441,819 which are hereby incorporated by reference in their entirety. However, the use of a higher pressure differential should be balanced against the larger parasitic energy demand associated with providing pressurized reactant. Other methods employed include use of a temperature rise from inlet to outlet to remove water with a reduced parasitic load.
Various flow field designs have been employed in fuel cell constructions. For instance, flow field plates have been employed which comprise a plurality of substantially straight parallel channels (for example, General Electric and Hamilton Standard LANL No. 9-X53-D6272-1 (1984)). The inlet and outlet ends of the channels are connected to inlet and outlet manifolds respectively. There are several advantages associated with substantially straight channels. For example, one advantage is that, compared to non-linear channels where there may be eddies near bends in the channel, straight channels provide less places for water to accumulate in the channels. Another advantage is that there is less turbulence in fluids flowing in the channels since there are no corners. Further, plates with straight parallel channel flow fields may be easier to manufacture than those with more complex shaped flow fields. Such flow fields however do not have a substantial pressure differential between adjacent channels.
Flow field plates have also been employed which comprise one or more parallel serpentine channels. The use of serpentine channels allows the channel length to be increased and hence allows the pressure differential between inlet and outlet to be increased without having to reduce the channel cross-sectional area; see for example U.S. Pat. No. 5,108,849, which is incorporated herein by reference in its entirety. Flow field plates employing serpentine channels typically have a pressure differential between adjacent channels or channel portions. This pressure differential arises from the presence of bends in the channels which results in adjacent channel portions being somewhat different in length from the inlet manifold.
Flow field plates have also been employed which comprise interdigitated inlet and outlet channels for reactant gases; see for example U.S. Pat. No. 5,641,586. The inlet channels may be connected to an inlet manifold and the outlet channels connected to an outlet manifold. However, in U.S. Pat. No. 5,641,586 the inlet channels and the outlet channels are separate and the reactant gas also traverses a macroporots material between them. Proper reactant distribution in an interdigitated flow field plate relies on pressure differentials between inlet channels and outlet channels, but there is no teaching or suggestion that it is advantageous for the inlet channels to have pressure differentials among themselves.
A variety of other flow field designs and/or modifications of the preceding have been proposed in the art in order to improve water management (water removal) and/or reactant distribution to the electrodes and hence to improve fuel cell performance. Generally though, the individual channels themselves are uniform over most of their length and thus the resistance to flow in the individual channels is uniform over most of their length. (Exceptions however include the short channel length portions at bends in a serpentine flow field or locations in which a channel branches into two or more channels.)
Increasing the pressure differentials between adjacent reactant channels in a fuel cell flow field can provide for improved fuel cell performance. The performance improvement may result from either improved water management or improved reactant distribution to electrodes in the fuel cell. The pressure differentials serve to drive reactant into and/or water out of regions not in direct fluid contact with the channels themselves (for example, regions of a diffusion layer that are supported by and in direct contact with the landings that separate the channels in a typical flow field plate). The pressure differentials may be increased by engineering adjacent channels such that the resistance to reactant flow differs locally along their length.
Flow fields are employed to supply fluid reactants to the electrochemically active areas of the fuel cell. In flow fields which comprise at least two adjacent fluid distribution channels and in which each of the two adjacent channels is connected at an inlet end to an inlet manifold, the pressure at the inlet ends of both channels is essentially the same. In a like manner, in flow fields in which each of the two adjacent channels is connected at an exhaust end to an outlet manifold, the pressure at the outlet ends of both channels is also essentially the same. Local pressure differentials may be increased between adjacent channels (including an increase from zero by inducing a pressure differential) over a substantial portion of channel length by using a channel design in which the resistance to flow of the fluid reactant along at least one of the two adjacent channels varies from inlet to outlet (that is, the resistance is not constant over the entire channel lengths) and in which the resistance to flow of the fluid reactant differs between each of the two adjacent channels over that channel length portion. Preferably, a channel design is selected such that pressure differentials are induced or increased over most of the length of the channels.
Preferably, the resistance to flow in the channels is varied by varying a geometric characteristic of the cross sections of the channels. For instance, flow resistance may be varied by varying the width, depth, or other shape characteristic of the channel cross section. Thus, pressure differentials may be induced or increased by designing flow field channels such that the cross sections of adjacent channels differ locally along most of the channel length. Preferably, the channel design still provides for essentially similar mass flow of fluid reactant in each channel.
For instance, a pressure differential may be induced between two otherwise straight parallel adjacent channels by decreasing the width of one of the two channels monotonically from its inlet end to its outlet end and by doing the opposite (that is, increasing the width monotonically) in the other channel. The term monotonically, as used herein, does not necessarily mean constantly or uniformly. For example, if a channel decreases in width monotonically, it means that it does not increase in width. Similarly, a pressure differential may be induced by decreasing the channel depth monotonically in one channel and by doing the opposite (that is, increasing the channel depth monotonically) in the adjacent channel.
The variation in channel cross section need not be monotonic however. Instead, the cross sections in one or both of two adjacent channels may vary cyclically and also result in increased local pressure differences over most of the length of the channels. For instance, the geometric characteristic of one of two adjacent channel cross sections (for example, width and/or depth) may vary cyclically in some way with channel length while that of the other channel varies cyclically with the same period but out of phase with its neighbor. Preferably, the other channel varies half a cycle out of phase with its neighbor. The cyclical variation may simply involve varying the geometric characteristic of the cross sections between two discrete values (for example, two different widths). Alternatively, a more complex repeating shape for the cross section may be employed, such as a repeating hour glass shape.
The performance of flow fields comprising two or more parallel channels may be improved in this way. In prior art flow fields with substantially linear, parallel, uniform channels, there may be no significant local pressure differential between adjacent channels. However, by appropriately varying flow resistance in the channels, pressure differentials may desirably be increase in such flow fields. In prior art flow fields with serpentine, uniform channels, there may be some local pressure differential between adjacent channels. However, by appropriately varying flow resistance in these serpentine channels, the pressure differentials may desirably be increased. To some extent, improved flow fields may additionally have interconnections between adjacent channels. Interconnections however may reduce the pressure differentials otherwise obtained by varying the flow resistance of adjacent channels.
Some embodiments of the present improved flow fields may comprise a plurality of fluid distribution channels. Pressure differentials may be induced or increased between each pair of adjacent channels in a manner similar to that for a single pair of adjacent flow field channels. For instance, this may be accomplished by having every other channel be of the same design.
The improved flow fields are particularly suitable for use in solid polymer electrolyte fuel cells supplied with gaseous reactants. The flow fields may be incorporated either in a flow field plate or integrated into a fluid diffusion layer. Fluid flow resulting from increased pressure differentials between adjacent channels may take place within a porous fluid diffusion layer or through porous flow field landings.